A major problem with many microwave systems, such as digital instantaneous frequency measurement (DIFM) devices, is the size and cost of the precision delay lines required. Currently, hermetically sealed, stainless steel jacketed, semi-rigid coaxial transmission lines with silicon dioxide as the dielectric material are used. This provides a low dielectric loss and a temperature stable delay; however, in a typical DIFM configuration, the coaxial delay line occupies a large portion of the unit's total volume and accounts for a significant portion of the unit's weight.
Because of the size of these delay line assemblies, they are usually manufactured as a separate subassembly and attached via coaxial connectors to the rest of the DIFM circuit. The coaxial cable must be coiled in order to fit the delay line into a restricted space. The coiled cable assembly must be potted in order to make the delay line rigid and avoid variations in electrical characteristics due to motion of the cable. The manufacture of such coaxial delay line assemblies is thus a labor intensive operation. Further, because the coaxial delay lines have to be coiled to conserve space, they must be made with a dielectric that can support the center conductor during the coiling operation. Thus, they do not permit the use of air as the dielectric. An air dielectric is advantageous in precision delay lines because the properties of solid dielectric materials vary significantly with temperature and with frequency.
Microwave circuits can be formed using flat, layered structures such as stripline. A stripline circuit is a pattern of flat conductors sandwiched between a pair of flat, sheet-like ground plane conductors and spaced from the ground plane conductors by a pair of intervening sheets of dielectric material. U.S. Pat. Nos. 4,394,633, 4,394,630, 4,375,054, and 3,621,478 show various stripline structures.
Traditional stripline structures, however, have not been suitable for high frequency, precision delay lines. In typical prior art stripline structures it is difficult to maintain the TEM transmission modeas the dominant mode at frequencies above 18 Ghz.
It is an object of the present invention, therefore, to provide a microwave transmission line that is suitable for use as a precision delay line while being both compact in size and low in manufacturing cost.
It is a further object of the invention to provide a precision microwave delay line wherein the dielectric may be air.